If your AC is working but just a little low, you can use those recharge kits/bottles sold at most auto parts stores. However, if you have no working AC, it's better to take it to an AC shop and let them recharge the system (and potentially spot any leaks) than to spend money on those recharge kits. Most AC shops will refill up to two pounds of refrigerant for about $70, versus spending $30-35 on a 1-pound recharge bottle that may or may not be enough. And if you do have a problem (like a leak) you just wasted your money with those recharge kits.